A path flanked by olive trees

Plants that changed the world: olive tree

Elena Gillies looks at a tree that has been a long-standing source of sustenance

Olives are ancient. Archaeological evidence suggests that wild olives were being gathered from around 19,000 BCE.

The olive tree is a slow-growing evergreen characterised by a gnarled trunk from which grey fissured branches sprout white flowers and fleshy grey-green narrow leaves. Trees can reach over 15 metres in height and live for hundreds of years in the right conditions. The olive itself is a drupe or stone-fruit with a single, centrally located seed (pit) surrounded by edible pulp.

Where olives are grown

These squat, oval-shaped fruits that herald from the Olea europaea tree have long been synonymous with the Mediterranean, where they thrive in the muggy winters and arid summers.

Olive trees are grown in eight EU countries including Spain, Italy and Greece. They also grow in Syria, Iran and California. In 2010, it was estimated that olive groves covered an area of 5 million hectares in the EU.

How olives are grown

There are two main methods of growing olives: traditional and modern. Traditional processes involve unirrigated mountainous or hilly landscape. Modern methods involve irrigation and mechanisation. In traditional growing, labour represents half the production costs, so olive growing is often a significant feature of cultural heritage across the Mediterranean.

Olives will darken from yellow to a deep purple as they ripen; at what stage they will be harvested depends on whether they will be used for oil or eaten whole. Taken from the tree, olives are extremely bitter and almost inedible. So, before eating, they are typically cured, in water, brine or oil.

The hard wood of the tree is also highly valued by woodworkers, while the leaves are frequently used in medicinal teas.

Oil-rich fruit

Olive oil is arguably the most famous product of the tree, with its myriad of uses – from fuel for oil lamps to cosmetics to kitchen staple to anointing oil in certain religious ceremonies. For example, the Oil of Catechumens used in Roman Catholicism is traditionally made from very pure forms of olive oil.

Olive oil is often regarded as healthier oil owing to its high content of monosaturated fats and as such is a staple ingredient of the so-called Mediterranean diet. Olive oil is predominantly composed of triglycerides, lipids consisting of one glycerol molecule bonded to three fatty-acid molecules. 

You have probably noticed on the supermarket shelf that there are a variety of grades for olive oil. When an olive oil is a ‘virgin’ blend, it simply means that the oil has been extracted from the olive by means (traditional or modern) that haven’t led to changes in the oil. ‘Extra virgin’ is often seen as a premium oil judged by its taste, colour, smell and acidity, and contains no more than 0.8 grams of free oleic acid per 100 grams of product. Oleic acid is an abundant fatty acid that occurs naturally in animal and vegetable fats and is present in the tricyglerides – but it’s the free oleic acid content that the oil is judged on.

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Further reading

About this resource

This resource was first published in ‘Plants’ in May 2016.

Ecology and environment
Education levels:
14–16, 16–19, Continuing professional development